The basic synaptic organization of the olfactory bulb (OB) has been known for several decades, but we continue to lack insight into the properties of these synaptic circuits and their underlying molecular determinants. The structural and molecular properties of axodendritic (A:D) synapses have been studied elsewhere in brain, but little work has been done on the A:D and dendrodendritic (D:D) synapses in the OB. Indeed, since the first descriptions of these synapses in the early '70s, there have been few reports on their structural/molecular properties. This paucity of information conflicts with the growing interest in the physiology of A:D and D:D synapses in the OB and their mechanisms of regulation. The current proposal has 3 specific aims that will generate new insights into the architectural/molecular properties of D:D synapses. In Aim 1 immunohistochemical techniques at both the confocal and electron microscopic levels will establish the distribution of synaptic vesicle associated proteins at A:D and D:D OB synapses. We hypothesize that dendritic architecture may lead to the expression of unique members of the vesicle cascade at D:D synapses. In Aim 2 we propose to use 3D reconstructions to examine the morphometric features of these synapses and test the hypothesis that the characteristics of the synapses are correlated. In Aim 3 we will examine critically the development of reciprocal A:D and D:D appositions in the OB at the ultrastructural level. Early studies suggested that the Gray Type 1 synapse may form first, followed by the Gray Type 2. However, this was based on single sections and morphometric statistics rather than the careful systematic 3D reconstructions that we propose. Similarly, while the role of functional activity in the development of the OB remains controversial, no studies have examined development at the ultrastructural level. In sum, we believe that these studies will provide significant new insights into the molecular mechanisms and vesicle proteins that regulate synaptic function at both A:D and D:D synapses in the OB. [unreadable] [unreadable]